The present invention relates to optical scanners and more specifically to an optical scanner having a power efficient lens.
Optical scanners are well known for their usefulness in retail check out and inventory control. Optical scanners generally employ a laser diode, the light from which is focused and collimated to produce a scanning beam. An optical transceiver directs the beam against a plurality of stationary mirrors, and collects the beam after it is reflected by a bar code label. A motor rotates the optical transceiver, and a detector receives the returning beam. The pattern produced by such a scanner is characterized by lines oriented at various angles to one another. An example of an optical scanner can be found in commonly assigned U.S. Pat. No. 4,971,410, entitled, "Scanning and Collection System for a Compact Laser", issued Nov. 20, 1990, to Wike, Jr. et al. This patent is hereby incorporated by reference.
In addition to lenses that focus, some optical scanners employ lenses that collect light after it has reflected from the object scanned. For example, commonly assigned U.S. Pat. No. 4,797,551, entitled "Compact Laser Scanner Optical System", issued Jan. 10, 1989, to Ferrante discloses an optical scanner employing a bifocal lens which focuses reflected light beams at a photodetector and which has a lenslet portion which focuses a laser beam onto a bar code label. This patent is hereby incorporated by reference.
The laser diodes used in optical scanners exhibit an undesirable characteristic. Unfocused laser light produces a scanning ellipse, having two propagation axes for power, one axis diverging around thirty-five degrees and the other axis diverging around ten degrees.
Known focusing techniques for optical scanners, such as the scanner disclosed in Wike, Jr., employ a single spherical focusing lens followed by a circular aperture. The circular aperture determines the size of the laser beam projected at a reference plane by separating the central portion of the focused light from the outer portion of the focused light. However, this method necessarily entails a loss of as much as eighty-five percent of the light's power as the diameter of the circular aperture must be limited to maintain the optimum "F" number for a proper beam profile about the primary focus.
Focusing techniques for collected light, such as those disclosed in Ferrante, position a photodetector at a fixed distance from the collecting lens. Thus, a bar code label must be positioned at a particular distance from the scanner in order for the collecting lens to focus the reflected light at the photodetector. Variation of the bar code label from the optimum distance causes reflected light to focus away from the photodetector and thereby reduces the amount of light received by the photodetector. Also, the solid angle subtended by the collecting lens tends to vary with item position. It is desirable to maintain a constant solid angle and thus a constant collection light signal for varying item distances.
Therefore, it would be desirable to produce optical scanners having power efficient lenses: an optical scanner having a focusing lens, capable of refracting diverging light through a circular aperture, and an optical scanner having a collecting lens, capable of collecting and focusing light at a photodetector for various bar code label distances from the scanner.